This invention pertains to the coagulative recovery of polysulfone resins from solutions thereof and more particularly to the use of aliphatic hydrocarbon non-solvent to effect coagulation.
One of the modes of preparing high polymers is by the solution polymerization of the corresponding monomer or monomers. It is known that the recovery of normally solid organic polymers from solution can be accomplished by non-solvent coagulation, i.e., by treating the solution with an excess of a liquid which is a non-solvent for the polymer but which is miscible with the polymer solvent. Agitation is usually required to facilitate mixing and to encourage the formation of normally solid polymer particles which can be isolated from the liquid phase without complications such as the formation of emulsions.
Other methods that have been used for the recovery of normally solid polymers from solution include precipitation induced by cooling the solution or by concentration of the solution by partial evaporation of the solvent. Removal of the solvent can also be induced by stripping at reduced pressures and/or elevated temperatures, melt recovery and spray precipitation.
The use of mechanical devolatizers to dry off residual solvents from recovered polymers leads to some thermal and shear degradation of the polymers and is often plagued by mechanical failures, seal strains and the like. Unfortunately, polymers, such as, normally solid polyarylene polyether polysulfone thermoplastic resins which are among the most difficult to rid of residual solvents, are also among the worst offenders in causing failure of mechanical processing equipment.
All of the above-described techniques have serious drawbacks. Precipitation and stripping are both expensive and time consuming and are usually limited to laboratory scale operations whenever possible. Melt recovery is complicated by mechanical failure of the equipment, and by degradation of the polymer.
The use of mixed solvents is complicated by the difficulty of solvent removal from the polymer particles during drying of the precipitated resin. It is well known in the art that polymers in general, regardless of their chemical make-up, retain solvent to varying degrees when isolated by prior art methods due to various causes, as for example, adsorption and mechanical or physical entrapment. Entrapment of solvents occurs with polymer structures ranging from the highly porous to the non-porous.
The effects of solvent retention by polymers are notorious and are reflected not only in deficient physical and mechanical properties but also in fabrication difficulties and in longer range manifestations such as polymer degradation or instability which is particularly serious in articles fabricated from the polymers. Regardless of the method used, it is often desirable to limit the amount of residual solvent retained in the recovered polymer to less than about 0.1%. In the case of polyarylene polyether polysulfone thermoplastic resins, it is preferred that the recovered resin contain less than about 500 parts per million (ppm) of residual solvent or any other liquid.
As the glass transition temperature of the polymer increases, the ease of recovery from solution by devolatilization methods decreases. The polyarylene polyether polysulfone thermoplastic polymers are particularly susceptible to this problem.
Another factor which complicates any system of polymer recovery from solution is the porosity and pore structure of the solid polymer particles. It is essential that the solvent/non-solvent mixture left within the precipitated resin maintain access to the external solution via the pore structure and that the pore walls do not collapse while removing the binary solution of solvent and non-solvent, as, for instance, by a drying operation; otherwise, entrapment of residual liquids ensues.
It is therefore an object of this invention to provide a suitable method for recovering normally solid, polyarylene polyether polysulfone thermoplastic resins from their solutions.
It is another object to recover normally solid polyarylene polyether polysulfone thermoplastic resins at temperatures as close to ambient temperatures as possible.
It is yet another object of this invention to recover normally solid polyarylene polyether polysulfone thermoplastic resins without entraping the binary solution of solvent and non-solvent either during precipitation or by collapse of the pore walls during drying.
It is still another object of this invention to recover dry, normally solid polyarylene polyether polysulfone thermoplastic resins containing less than 500 ppm of residual liquids.